Feedback control of a drive wheel drive torque based on a difference between wheel speed and chassis speed such that the drive wheel does not slip excessively when a vehicle starts or accelerates is disclosed for example in the following references.
(1) Tan and Chin: Vehicle Traction Control: Variable Structure Control Approach. Trans. of ASME Dynamic Systems, Measurement and Control, 113 223/230 (1991)
(2) Chin, William, Sidlosky, Rule and Sparschu: Sliding-Mode ABS Wheel-Slip Control Proc. of Am. Control Conf. 1/5 (1992)
In the devices described in these references, a function having vehicle speed and wheel speed as arguments is set, and the slip ratio between the road surface and a tire is controlled to a target value by changing over a drive torque according to whether the value of the function is positive or negative.
However, if there is oscillation of the drive torque in this device due to a response delay with respect to a throttle operation of the engine which generates the drive torque, it may not reach a desired value.
The control device moreover comprises a digital processor. When the chassis speed and wheel speed are sampled with a limited sampling interval, a response delay occurs in the control of drive torque due to the time required for calculation of the function, and oscillation of the drive torque or slip ratio tends to occur.
To suppress such an unfavorable oscillation, the characteristics of the function may be set so that there is no sudden change-over of drive torque, and the drive torque varies smoothly from a maximum value to a minimum value. However when the drive torque is varied intermittently in this way, the control error increases, and the target value may not correspond precisely with the actual value of the slip ratio.